The "holy grail" of transplant biologists is to achieve long-term survival of an allogeneic transplant without widespread immunosuppressive treatment of the recipient. Such transplantation tolerance is regularly achieved in nature during pregnancy. Several studies have provided evidence showing that adult individuals display some degree of immune tolerance towards noninherited maternal antigens (NIMA). However, the mechanisms underlying this phenomenon are still unknown. A thorough understanding of this issue is likely to provide the basis for the design of selective immune therapies mimicking this natural form of transplantation tolerance. We have designed a mouse transgenic model to elucidate the mechanisms underlying the tolerance of offspring to NIMA. In this model the NIMA is a MHC class I molecule, Kb and the offspring express a T cell receptor transgene anti-Kb. Non-NIMA exposed mice rejected Kb+ heart transplants in 10 days. In contrast, Kb+ cardiac transplants enjoy long-term survival in NIMA mice. No deletion of anti-Kb T cells and no reduction of anti-Kb TCR surface expression was found in NIMA mice, thereby excluding a deletional mechanism. In contrast to control NE mice, T cells from heart-transplanted NIMA mice exposed in vitro to Kb+ stimulators secreted no inflammatory cytokines (IL-2, gIFN) but produced high levels of IL-4 cytokines. Finally, the tolerogenic effect observed in NIMA mice was abrogated by in vivo depletion of CD4+ (but not CD8+) T cells. Therefore, mice that have been exposed to NIMA Kb MHC class I during pregnancy and/or breast feeding have become tolerant to this alloantigen via an active regulatory process involving CD4+ T cells. The main objectives of this proposal are to elucidate: 1) the mechanisms by which the fetus and/or newborn mouse becomes exposed to NIMA and, 2) the mechanisms by which NIMA-exposed mice are rendered tolerant to these alloantigens and accept corresponding heart allotransplants. To address these questions, we plan the following specific aims: Specific aim 1. To detect and determine the nature of maternal cells present at different time points in fetuses, newborns, weanlings, as well as young and adult offspring. Specific aim 2. To investigate the anti-Kb alloresponse in NIMA mice. Specific aim 3. To elucidate the mechanisms underlying T cell tolerance to NIMA. The implications of the NIMA effect for a variety of applications are numerous, and include cord blood stem cell transplantation, cadaveric organ transplantation as well as in non-transplant fields such as autoimmunity, and development of antitumor vaccination approaches using "self antigenic peptides, both of which may benefit from an understanding of the basic mechanisms of NIMA tolerance.